In general, when a shell-type milling cutter is compared with such an integral structure type milling cutter being integrally composed of a cutting edge portion and a shank portion, the shell-type milling cutter is inferior to that of the integral structure type in all the points of performance such as rigidity, machinability, strength, accuracy of finishing, wear proof and the like, since an area where the cutting edge portion of the milling cutter is in close contact with the fitting shaft of a cutter arbour is small and a degree of such close contact is low. On the other hand, if size of a milling cutter itself is large, the manufacture of such an integral structure type milling cutter becomes expensive, besides it is very uneconomical because the worn cutting edge portion of which must be replaced by a fresh one together with a fresh shank portion despite unnecessary replacement of the old shank portion which has not yet been worn, and for this reason, a shell-type milling cutter only the edge portion of which is exchangeable and which is inexpensive and easily manufactured has been used at the present time. In this connection, as a method for attaching such shell-type milling cutter to its cutter arbour, there has heretofore been, as shown in FIG. 1, such a method that a fitting shaft (31a) of a cutter arbour (31) is inserted into a fitting hole of a shell-type milling cutter (11), and further the milling cutter (11) is fixed to the cutter arbour (31) by means of a bolt (5). In this method, however, when a cutting work is conducted, the bolt (5) becomes loose due to vibration derived from the cutting, so that a degree of close contact of the milling cutter (11) to the cutting arbour (31) becomes poor. In this respect, when a rotational frequency or an amount of cut is increased in order to cover such defects as described above, vibration and the like produces, and as a result there arises a further disadvantage such that a life of the shell-type milling cutter (11) is remarkably shortened, accuracy in the finishing becomes inferior so that the rotational frequency and the amount of cut are restricted, whereby an improvement of the cutting efficiency is significantly impeded. Furthermore, there is another method for attaching a shell-type milling cutter to a cutter arbour wherein the outer circumference (32a) of a fitting shaft of the shell-type cutter arbour (32) as well as a fitting hole (12a) of the shell-type milling cutter are defined with a left-handed screw thread, respectively, as shown in FIG. 2, and both the members are threadedly engaged with each other. According to this method, when a cutting work is made, the screw is automatically fastened to increase a degree of close contact of the milling cutter (12) to the cutter arbour (32), so that high-speed rotation and heavy-duty cutting becomes possible.
In this case, however, such cutting resistance resulted is added to the threaded portion as a load, so that there are such serious disadvantages that the threaded portion is completely fastened with increase of the cutting resistance, and in the most serious situation, seizure phenomenon occurs and it makes impossible to separate the shell-type milling cutter (12) from the cutter arbour (32), whereby exchange of the shell-type miling cutter (12) becomes impossible, and the circularity of which becomes incorrect.
In view of the above, the present invention has been made to eliminate disadvantages involved in the aforesaid conventional shell-type milling cutters as well as a method for attaching such shell-type milling cutters. Namely, an object of the present invention is to provide a shell-type milling cutter and a method for attaching the shell-type milling cutter having the undermentioned advantages. The shell-type milling cutter according to the present invention is obtained by the following manner. To a fitting shaft of a cutter arbour the extreme end of which is formed into a straight stepped portion, a milling cutter on which has been bored a tapered hole having somewhat larger diameter than that of said fitting shaft is loosely fitted. Before inserting two divided tapered members into a gap defined by said fitting shaft and said tapered hole, each projection of said tapered members is fitted into a concave groove defined on a bolt to engage them with each other. These tapered members are forcibly inserted into the gap defined by said fitting shaft and said tapered hole while maintaining said engagement state, whereby said milling cutter is secured to said fitting shaft while keeping said forcible insertion. As a result, an area where the shell-type milling cutter is allowed to be in close contact with the cutter arbour increases and in addition, since the projections of the tapered members have been fittingly engaged with the concave groove of the bolt, accuracy in shaft center is very well, rigidity and attachment strength increase, so that the resulting shell-type milling cutter makes high-speed and heavy-duty cutting possible. Consequently, cutting efficiency of the shell-type milling cutter of the present invention becomes substantially the same with that of an integral structure type milling cutter. Moreover, a milling cutter can be very easily exchanged in the present invention.